Flip chip is a method for interconnecting semiconductor devices, such as integrated circuit (IC) chips to circuitry in a package substrate, using solder bumps deposited onto the IC pads. The solder bumps are deposited on the IC pads on the top side of the wafer during wafer processing. After singulation, the IC chip is flipped over so that its top side faces down, and aligned so that its pads align with matching pads on the package substrate, and then the solder is flowed to complete the interconnect. An underfill material is introduced into the space between the IC and substrate, surrounding the solder balls. The flip chip method uses substantially less space than wire bonding, enabling fabrication of a chip-scale package.
Flip chip package substrates must have very flat surfaces, which may be difficult to maintain as the boards heat and cool. Also, the solder ball connections are very stiff. If the IC and the package substrate have different coefficients of thermal expansion (CTE), differential expansion occurs when the package is heated and cooled. The differential expansion places large stress on the solder ball connections, which may cause the connections to crack or delaminate from the substrate, or cause warpage of the package substrate.
Additional challenges present themselves when a flip chip package is included in a three dimension (3D)-IC configuration, such as a package on package configuration. In a package on package configuration, two IC packages, such as an application specific IC (ASIC) and a memory package are installed one on top of the other. For example, the top package may be larger than the bottom package, and may have an array of solder balls around its perimeter for forming connections to the package substrate of the bottom package. Configurations such as package on package increase the likelihood of package substrate warpage.